Because the molding of glass optical elements is done by compression rather than injection (as is utilized in plastic molding) a precursor metered amount of glass, generally referred to as a preform, is required. There are two fundamental shapes of preforms required which generally parallel the two fundamental finished lens shapes. For negative lenses, plano-plano preforms usually will be sufficient. These can be fabricated in high volume relatively inexpensively by grinding and polishing. For positive lenses, a ball (sphere) or ball-like lump of glass is needed. The basic constant when molding positive or negative lenses is that the molds must touch the softened glass at the center first and then press out to the edges to avoid wrinkles and voids in the finished lens element.
Various methods and apparatus for the compression molding of glass optical elements are known in the prior art. With these methods and apparatus, optical element preforms sometimes referred to as gobs are compression molded at high temperatures to form glass lens elements. The basic process and apparatus for molding glass optical elements is taught in a series of patents assigned to Eastman Kodak Company. Such patents are U.S. Pat. No. 3,833,347 to Angle et al, U.S. Pat. No. 4,139,677 to Blair et al, and U.S. Pat. No. 4,168,961 to Blair. These patents disclose a variety of suitable materials for construction of mold inserts used to form the optical surfaces in the molded optical glass elements. Those suitable materials for the construction of the mold inserts included glasslike or vitreous carbon, silicon carbide, silicon nitride, and a mixture of silicon carbide and carbon. In the practice of the process described in such patents, a glass preform is inserted into a mold cavity with the molds being formed out of one of the above mentioned materials. The molds reside within a chamber in which is maintained a non-oxidizing atmosphere during the molding process. The preform is then heat softened by increasing the temperature of the mold to thereby bring the preform up to about 100.degree. C. above the Glass Transition Temperature (T.sub.g) for the particular type of glass from which the preform had been made. Pressure is then applied by the mold to force the preform to conform to the shape of the mold. The mold and preform are then allowed to cool below the transition temperature of the glass. The pressure from the mold is then relieved. The temperature is lowered further and the finished molded lens is removed.
One of the major factors which greatly effects the economics and quality of precision glass molding of optical elements is the fabrication of the precursor quantity of glass referred to herein as a preform. A preform may be distinguished from a blank such as those used in the conventional grinding and polishing operations for the manufacture of a lens. A blank has glass removed to become a finished part. A preform is merely reshaped in the molding process and it is not necessary to remove glass from the preform. This requires that the preform have a very good surface quality as well as internal quality. There are two principal methods used to fabricate preforms. The first is to grind and polish the preform to lens type quality in a shape that allows it to be further reshaped to a finished lens through the molding operation. Although this is the most straight forward method, it is also the most costly. The second technique is to produce the preform through some type of molten glass gobbing process. One such process is taught in U.S. application Ser. No. 08/725,382 entitled, "Method and Apparatus for Making Optical Preforms with Controlled Peripheral Edge Wall Geometry". A variant of this method is taught in U.S. Pat. No. 5,709,723 to Gearing.
The prior art fails to teach a method for producing continuous rods or sticks of glass having high surface and internal quality which can be cut to length to produce preforms of predetermined volumes with both good surface and internal quality. In such manner, the production of preforms becomes substantially continuous.